Setup, so nobody has to ask: 97.9% · Medutest · 98.8% · Janoshik.
Both of these get recommended confidently by different people, which suggests neither is obviously right.
My constraints are cost, measurement resolution and how much handling I am prepared to do — in roughly that order.
Is there a defensible reason to prefer one, or is this a coin flip?
97.9 and 98.8 are 0.9 percentage points apart, which sounds small until you restate it as impurity. Medutest is leaving 2.1 per cent of the detected area unassigned and Janoshik 1.2 per cent — a factor of 1.75 between them. The impurity fraction is the quantity that moves when a method changes, and it is the one worth arguing about; the headline is just its complement. Comparable means same column chemistry, same gradient slope, same detection wavelength, same integration convention. Until you have those four from both laboratories, 97.9 and 98.8 are two measurements of slightly different quantities that happen to share a unit.
To be exact about it, the single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
| Component | Typical share | Counted in purity? | Counted in content? |
|---|---|---|---|
| Target peptide | 88–94 % | Yes, as main peak | Yes |
| Related impurities | 1–3 % | Yes, as other peaks | No |
| Counter-ion (TFA or acetate) | 2–8 % | No | No |
| Residual water | 2–6 % | No | No |
| Bulking agent, if present | 0–40 % | No | No |
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 26 Mar 2025 by mz_4113 — clarified the distinction between purity and content
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsStart from what the detector sees, because that tells you what the number means.
Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.
The relevant detail is that mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
In practice, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
Integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.
Specifically, detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.
Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.
Compare purity within a single laboratory on the same method, never across laboratories.
Purity is a method-dependent figure, and that is not a limitation of the measurement, it is a property of what the measurement actually answers.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.
If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.
Identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.